Internal combustion engines have been used to produce power and drive machines for over a century. Historically, gasoline and diesel have been preferred fuel choices because they were abundant, inexpensive, and easy to store. While natural gas has been used as a fuel for vehicles for over fifty years, widespread use has been curtailed for various reasons including fuel storage density, infrastructure, availability of fuel, and capital costs that are generally higher compared to conventional liquid-fuelled vehicles. However, alternative fuel choices are receiving renewed attention because of several factors, including changing economic conditions, the desire to reduce pollution, and the desire to reduce dependency on diminishing and increasingly expensive oil resources.
There are numerous advantages to substituting liquid fuels with natural gas or other gaseous fuels that are combustible in an internal combustion engine and which are in the gaseous phase at atmospheric pressure and temperature. Natural gas is itself a mixture of combustible gases, but it is primarily methane. Other gaseous fuels include ethane, propane, and other lighter flammable hydrocarbon derivatives as well as hydrogen and mixtures thereof. For example, mixtures of hydrogen and natural gas have been used as a fuel for internal combustion engines, and such mixtures are sometimes referred to as “HCNG”. Compared to conventional liquid fuels, the gaseous fuels described herein are generally cleaner burning and can be produced from renewable sources. Natural gas is one of the most abundant gaseous fuels available today and in the examples described herein, natural gas is named as the fuel but suitable gaseous fuels that are combustible could also be employed in the internal combustion engines described herein.
In the past, most natural gas fuelled vehicles (“NGV”) were naturally fumigated, that is, natural gas was not introduced into the cylinders directly, but rather by being injected into the intake manifold, where the fuel mixed with the intake air and was fed into the cylinders with the intake air, entering through the intake port and open intake valves. Compared to engines that inject the fuel directly into the combustion chamber late during the compression stroke, the injection pressure needed to overcome the pressure in the intake manifold is relatively low. Accordingly, the fuel supply system for a naturally fumigated NGV is relatively simple. Fuel is held in and supplied from a liquefied natural gas (LNG) storage tank with a working pressure just above the engine inlet pressure, or from compressed natural gas (“CNG”) cylinders through pressure regulators which reduce the pressure to the desired injection pressure. CNG is commonly stored at ambient temperatures at pressures up to 250 bar, but a disadvantage of CNG is its lower energy density compared to conventional liquid fuels or LNG and the relatively heavy weight normally associated with CNG storage tanks, which need to be designed to withstand the high storage pressures.
On the other hand, LNG is normally stored at temperatures of between about −160° C. and −130° C. and at lower pressures compared to CNG, for example, less than 10 bar and typically between about 2 and 8 bar, with an energy density that is about four times higher than that of CNG. LNG storage tanks provide an acceptable means for storing a sufficient volume of natural gas on board NGVs. However, using LNG as a fuel supply introduces some complications in the fuel handling and supply systems. One complication is that the liquefied natural gas must be warmed and converted into gaseous form before being supplied to the engine. Also, to store such fuel in liquefied form it must be kept at cryogenic temperatures, requiring the storage tank to be thermally insulated to reduce heat transfer into the storage space. However, because cryogenic storage tanks require structural elements and pipes for filling and dispensing the fuel, there is at least some amount of heat transfer into the storage space, and when heat is absorbed by the stored liquefied gas, some of it can be converted to vapor which has a lower density than the liquefied gas, causing an increase in the storage pressure. This vapor is known as boil-off gas (“BOG”) and it needs to be re-liquefied or removed from the storage space to prevent the storage pressure from rising above the pressure limits of the storage tank. Some fumigated engines can take the BOG and inject it into the intake manifold or intake ports if the pressure of the BOG is higher than the intake air pressure.
Newer, more efficient engines for NGVs, referred to herein as high pressure direct injection (“HPDI”) engines, have been developed that inject the gaseous fuel at high pressure, late in the compression stroke to emulate the performance and efficiency of a diesel engine. The fuel injection pressure for HPDI engines is typically at least 200 bar. To raise the pressure of the stored liquefied gas from storage pressure to at least 200 bar a cryogenic pump is normally used. A suitable drive is required to drive the cryogenic pump in these pressure boosting systems. In vehicle applications, the drives are typically powered by an electric motor or hydraulically with power being transmitted through a working fluid to the pump drive. The hydraulic power in the latter case is typically derived from the vehicle engine which is itself being supplied with natural gas fuel originating from the LNG storage tank.
Because of the high injection pressures needed for HPDI engines, such engines are unable to use the BOG vented from the storage tank because the BOG pressure is too low and it is not efficient and therefore not practical to compress the vapor to the high pressures needed for direct injection into the combustion chamber. For HPDI engines, several approaches have been applied to reduce the amount of BOG, including reducing heat transfer into the storage tank, designing pumps that can pump both vapor and liquefied gas as disclosed in co-owned U.S. Pat. No. 5,884,488, introducing the BOG into the engine's intake manifold to offset some of the fuel that is injected directly into the combustion chambers, and by burning the BOG in a heater or some other non-engine apparatus. With some of these approaches there can still be times when a small amount of BOG cannot be directed to useful applications, resulting in the BOG being vented from the storage tank. There is a need to prevent the venting of BOG because this represents waste, while also posing emissions and safety concerns.